Place the onion, serrano pepper, garlic cloves, lime juice and cilantro into a blender. Pulse until chopped.
Add buttermilk, mayonnaise, cup sour cream, ripe avocado, salt, ground cumin, Mexican oregano, sugar, dark chili powder and black pepper. Cover and pulse until smooth. Add more buttermilk, if desired to thin.
Store chilled in an airtight container for at least 4 hours before serving.
Can be made up to 3 days in advance, shake well before serving. Serve as a dressing, condiment or dip.
Notes
This dressing can be prepared with light dairy products and mayonnaise, if desired.